Certain vehicle designs including hybrid electric vehicles (HEV) can selectively utilize different energy sources to optimize fuel efficiency. For propulsion, an HEV having a full hybrid powertrain can use either or both of an internal combustion engine and a high-voltage (HV) energy storage system (ESS). That is, a typical full HEV can be electrically propelled, usually immediately upon starting the HEV and at relatively low vehicle speeds. One or more HV motor/generator units (MGU) may alternately draw power from and deliver power to the ESS as needed. Upon launch or above a threshold speed, the engine can be restarted using the MGU or a smaller auxiliary starter motor, and then engaged with a transmission to provide the required vehicle propulsive torque to a set of drive wheels.
In contrast to the full hybrid powertrain, a mild hybrid powertrain typically lacks the capability of propelling the vehicle by purely electrical means, but nevertheless retains certain key design features of the full hybrid powertrain, e.g., regenerative braking capability for recharging the ESS using the MGU, as well as the capability of selectively shutting down or powering off the engine at idle. The capability of a HEV to selectively shut off and restart the engine when the vehicle is at a standstill, and/or when operating in a stabilized low-speed drive mode, provides a fuel-saving benefit relative to conventional vehicle designs. However, execution of an auto stop may not be efficient under all circumstances, and therefore under certain operating conditions the engine of an HEV may remain fueled and running at idle.